FIGURE 5.

Ca²⁺ stimulates NAADP synthesis in sea urchin sperm via base exchange. A, NAADP levels in sperm diluted into normal artificial sea water (n = 7). B, NAADP levels in sperm diluted into normal artificial sea water containing the Ca²⁺ ionophore ionomycin (5 μm) with (11 mm Ca²⁺) (n = 11) or (C) without (no added Ca²⁺ and 1 mm EGTA) extracellular Ca²⁺ (n = 3). D, NAADP levels in sperm diluted into normal (11 mm Ca²⁺) artificial sea water containing the Ca²⁺ ionophore ionomycin (5 μm) and the base exchange inhibitor nicotinamide (50 mm) (n = 6). E, summary of the NAADP levels following the indicated treatments. Data are the means ± S.E. of the mean based on all values after time zero from panels A–D (n = 3–11). Filled bars are significantly different from the control (p < 0.01, Dunnett's multiple comparisons test after one-way analysis of variance, p = 0.006, performed on the logged data). F, nicotinamide concentration-inhibition relationship for NAADP synthesis by base exchange monitored by addition of exogenous substrates. G and H, Michaelis-Menten plots demonstrating mixed inhibition of NAADP synthesis monitored by exogenous substrate addition.